The devices used to drive a floating circuit with a digital or logic signal, also known as level shifters, must not lose the logic signal in the variations caused by the charging and discharging of the parasitic input capacitor of the floating circuit while the voltage thereof shifts with respect to the reference ground. In order that the input node of the floating circuit may assuredly follow the variations of the driving signal which drives the level shifter, the parasitic capacitor must in fact charge and discharge with the same speed as the driving logic signal, and for this purpose the input node of the floating circuit must be connected to a low-impedance point.
A simple way of solving the problem consists in causing a current to flow in a resistor which has one terminal at the high floating voltage and the opposite terminal connected to the input of the floating circuit. However, the resistor must have a small value, for example 1 kohm, and the voltage drop required across the resistor in order to produce a logic signal on the floating circuit requires a high current and thus a considerable power absorption.
In order to avoid these undesirable continuous consumptions, one solution consists in providing a flip-flop at the input of the floating circuit, with a double pulse drive in two high-value resistors for switching the flip-flop. In this case, current consumption in steady-state conditions is low, but in order to maintain a low impedance even during switching it is necessary to provide current sources for rapidly charging the parasitic capacitor. This entails a certain circuital complexity, besides the fact that in any case the presence of the flip-flop entails the risk of errors on the phases of the logic signal in case of outside noise during rise and fall.